[PATCH RFC 04/17] KVM: guest_memfd: Add helper to attach resource provider file
From: Ackerley Tng via B4 Relay
Date: Fri Sep 25 2026 - 20:53:21 EST
From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
A refcount on the resource_fd is taken only to keep the reference stable
for guest_memfd to figure out the existence of a provider. The provider is
responsible for pinning anything required to keep the provider's ops and
the actual provided resource alive.
.release() is called when this guest_memfd inode is destroyed to allow the
provider to clean up.
Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
Documentation/virt/kvm/api.rst | 28 ++++++++++++++---------
include/linux/kvm_host.h | 2 +-
include/uapi/linux/kvm.h | 5 ++++-
virt/kvm/guest_memfd.c | 51 ++++++++++++++++++++++++++++++++++++++++--
4 files changed, 72 insertions(+), 14 deletions(-)
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 67f0f290797ab..b48df2827a2c3 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6470,7 +6470,9 @@ and cannot be resized (guest_memfd files do however support PUNCH_HOLE).
struct kvm_create_guest_memfd {
__u64 size;
__u64 flags;
- __u64 reserved[6];
+ __s32 resource_fd;
+ __u32 pad;
+ __u64 reserved[5];
};
Conceptually, the inode backing a guest_memfd file represents physical memory,
@@ -6492,15 +6494,21 @@ a single guest_memfd file, but the bound ranges must not overlap).
The capability KVM_CAP_GUEST_MEMFD_FLAGS enumerates the `flags` that can be
specified via KVM_CREATE_GUEST_MEMFD. Currently defined flags:
- ============================ ================================================
- GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file
- descriptor.
- GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during
- KVM_CREATE_GUEST_MEMFD (memory files created
- without INIT_SHARED will be marked private).
- Shared memory can be faulted into host userspace
- page tables. Private memory cannot.
- ============================ ================================================
+ ============================= ================================================
+ GUEST_MEMFD_FLAG_MMAP Enable using mmap() on the guest_memfd file
+ descriptor.
+ GUEST_MEMFD_FLAG_INIT_SHARED Make all memory in the file shared during
+ KVM_CREATE_GUEST_MEMFD (memory files created
+ without INIT_SHARED will be marked private).
+ Shared memory can be faulted into host userspace
+ page tables. Private memory cannot.
+ GUEST_MEMFD_FLAG_USE_RESOURCE Consume resource_fd as an external memory
+ provider or resource pool. When not set,
+ resource_fd must be '0'. When set, the provider
+ must be a supported resource (e.g. tmpfs mount
+ root directory). Currently, tmpfs resource pools
+ require noswap and huge=never mount options.
+ ============================= ================================================
When the KVM MMU performs a PFN lookup to service a guest fault, the fault will
always be consumed from guest_memfd, regardless of whether it is a shared or a
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 485f18454eb45..f4241c05650e9 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2588,7 +2588,7 @@ bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm);
static inline u64 kvm_gmem_get_supported_flags(struct kvm *kvm)
{
- u64 flags = GUEST_MEMFD_FLAG_MMAP;
+ u64 flags = GUEST_MEMFD_FLAG_MMAP | GUEST_MEMFD_FLAG_USE_RESOURCE;
if (!kvm || kvm_arch_supports_gmem_init_shared(kvm))
flags |= GUEST_MEMFD_FLAG_INIT_SHARED;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 8dff2fc1972e9..f4108bf7e3192 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1674,11 +1674,14 @@ struct kvm_memory_attributes2 {
#define KVM_CREATE_GUEST_MEMFD _IOWR(KVMIO, 0xd4, struct kvm_create_guest_memfd)
#define GUEST_MEMFD_FLAG_MMAP (1ULL << 0)
#define GUEST_MEMFD_FLAG_INIT_SHARED (1ULL << 1)
+#define GUEST_MEMFD_FLAG_USE_RESOURCE (1ULL << 2)
struct kvm_create_guest_memfd {
__u64 size;
__u64 flags;
- __u64 reserved[6];
+ __s32 resource_fd;
+ __u32 pad;
+ __u64 reserved[5];
};
#define KVM_PRE_FAULT_MEMORY _IOWR(KVMIO, 0xd5, struct kvm_pre_fault_memory)
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 00f3bd0cf21e6..0e0d7e93f4148 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -71,6 +71,12 @@ static inline void gmem_provider_invalidate_folio(struct gmem_inode *gi,
gi->provider_ops->invalidate_folio(gi->provider, folio);
}
+static inline void gmem_provider_release(struct gmem_inode *gi)
+{
+ if (gi->provider_ops && gi->provider_ops->release)
+ gi->provider_ops->release(gi->provider);
+}
+
#define kvm_gmem_for_each_file(f, inode) \
list_for_each_entry(f, &GMEM_I(inode)->gmem_file_list, entry)
@@ -984,7 +990,37 @@ static int kvm_gmem_init_inode(struct inode *inode, loff_t size, u64 flags)
return r;
}
-static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
+static int kvm_gmem_attach_resource(struct inode *inode, int resource_fd)
+{
+ struct file *resource_file;
+ struct inode *res_inode;
+ const struct guest_memfd_provider_operations *ops = NULL;
+ void *provider;
+
+ resource_file = fget_raw(resource_fd);
+ if (!resource_file)
+ return -EBADF;
+
+ res_inode = file_inode(resource_file);
+ ops = res_inode->i_sb->s_op->gmem_provider_ops;
+
+ if (!ops || !ops->attach || !ops->alloc_folio) {
+ fput(resource_file);
+ return -EOPNOTSUPP;
+ }
+
+ provider = ops->attach(resource_file);
+ fput(resource_file);
+ if (IS_ERR(provider))
+ return PTR_ERR(provider);
+
+ GMEM_I(inode)->provider = provider;
+ GMEM_I(inode)->provider_ops = ops;
+
+ return 0;
+}
+
+static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags, int resource_fd)
{
static const char *name = "[kvm-gmem]";
struct gmem_file *f;
@@ -1018,6 +1054,12 @@ static int __kvm_gmem_create(struct kvm *kvm, loff_t size, u64 flags)
if (err)
goto err_inode;
+ if (flags & GUEST_MEMFD_FLAG_USE_RESOURCE) {
+ err = kvm_gmem_attach_resource(inode, resource_fd);
+ if (err)
+ goto err_inode;
+ }
+
file = alloc_file_pseudo(inode, kvm_gmem_mnt, name, O_RDWR, &kvm_gmem_fops);
if (IS_ERR(file)) {
err = PTR_ERR(file);
@@ -1057,7 +1099,10 @@ int kvm_gmem_create(struct kvm *kvm, struct kvm_create_guest_memfd *args)
if (size <= 0 || !PAGE_ALIGNED(size))
return -EINVAL;
- return __kvm_gmem_create(kvm, size, flags);
+ if (!(flags & GUEST_MEMFD_FLAG_USE_RESOURCE) && args->resource_fd)
+ return -EINVAL;
+
+ return __kvm_gmem_create(kvm, size, flags, args->resource_fd);
}
int kvm_gmem_prepare_memory_region(struct kvm *kvm, struct kvm_memory_slot *slot,
@@ -1401,6 +1446,8 @@ static void kvm_gmem_destroy_inode(struct inode *inode)
{
struct gmem_inode *gi = GMEM_I(inode);
+ gmem_provider_release(gi);
+
mpol_free_shared_policy(&gi->policy);
/*
--
2.56.0.rc1.315.gc6ed9934b7-goog